It is already known to continuously produce pipes from reinforced plastics consisting of thermosetting resins such as unsaturated polyester resins reinforced with glass fibers. For example, U.S. Pat. No. 3,464,879 discloses a process for continuously producing a reinforced plastic pipe which comprises winding glass fibers impregnated with an unsaturated polyester resin around an endless belt which is helically wound on a rotating mandrel and which moves in the axial direction of the mandrel while rotating together with the mandrel, and passing the resin-impregnated glass fibers moving together with the endless belt through a curing zone, to thereby cure the unsaturated polyester resin. The reinforced plastic pipe so produced is removed from the endless belt after curing, and severed to predetermined lengths by a suitable cutter. The pipes obtained are used, for example, as water supply pipes, drainage pipes, or crude oil transporting pipes. In using a reinforced plastic pipe in these applications, a pipe connecting method is employed which comprises shaping one end of a pipe as a bell end and the other end as a spigot end, fitting the spigot end of one pipe into the bell end of another pipe, and sealing the space between them water-tight by a push-on type or O-ring type sealing gasket.
Various methods have been suggested for forming a bell element at one end of a pipe, and the methods disclosed in U.S. Pat. No. 3,784,667 and Japanese Pat. Publication No. 5024/76 are among them.
When a reinforced plastic pipe is produced by the continuous manufacturing method described hereinabove, the surface smoothness of the pipe is markedly impeded, and considerable unevenness occurs on its outside surface. If, therefore, one end of the resulting pipe is used directly as a spigot end and fitted into a bell end formed by the known method mentioned above, the adhesion between the spigot portion and the sealing gasket will become poor because of the unevenness of the outside surface of the spigot portion. This presents two problems. Firstly, the sealing gasket tends to be removed during the operation of fitting the spigot portion into the bell portion. Secondly, even when the sealing gasket is not removed, water leakage occurs at the uneven part of the spigot element.
One of the methods widely practiced heretofore to avoid these problems involves smoothing one end of a pipe to be used as a spigot by polishing. Generally, the polishing step in this method is performed batchwise, and therefore, causes the disadvantage that pipe production cannot be performed continuously. Furthermore, such method has the disadvantage that the polishing step is complicated and time-consuming and thus reduces productivity, and the polishing operation causes the scattering of dust which is likely to affect the health of the working personnel. Such method also suffers from a more serious defect in that the strength of the pipe is reduced because the polishing causes the thinning of the glass fiber-reinforced plastic layer at the outside surface of the spigot element. In the production of reinforced plastic pipes, the dual desire of maximizing the strength and rigidity of the pipes on one hand and of minimizing the cost of production on the other led, in many cases, to the employment of a sandwich-structure method wherein the outside surface layer and inside surface layer of a pipe which contribute mainly to strength are formed of an unsaturated polyester reinforced with a roving of glass fibers, and the intermediate layer which contributes mainly to rigidity is formed of an unsaturated polyester reinforced with silica sand which is available at low cost or a mixture of silica sand and a chopped strand of glass fibers. The thickness of these layers vary depending upon the diameter of the pipe and its required strength. Usually, the thickness of each of the outside surface layer and the inside surface layer is about 1 to 3 mm, and the thickness of the interlayer is about 10 to 20 mm. Shaving of the outside surface layer by a thickness of, say, 0.5 to 1 mm, by polishing therefore causes a serious reduction in its strength.
Japanese Pat. Publication No. 1944/74 discloses a method of polishing a reinforced plastic pipe continuously. According to this method, a reinforced plastic pipe whose curing has been completed in a first curing zone is moved to a polishing zone where the outside surface of the pipe is polished continuously. A solution of an unsaturated polyester resin is then sprayed onto the polished outside surface. The pipe is then passed through a second curing zone to cure the resin solution. This method requires two curing zones and has the defect of increased costs and space for the installation of equipment. Furthermore, a boundary surface occurs between the reinforced plastic layer cured first and the unsaturated polyester layer applied and cured later, and the mechanical strength of the pipe is insufficient.
U.S. Pat. No. 3,623,930 discloses a method for forming a spigot portion containing an O-ring slot at one end of a pipe by clamping a split ring mold configured to form a slot around the pipe, filling an uncured polyester resin into a space between the split ring mold and the pipe, and then curing it. This method, however, has the defect that the manufacturing steps are complicated, and the operation must be performed batchwise.
It is an object of this invention to provide a method which eliminates the defects of the prior methods for forming a spigot element in a reinforced plastic pipe, and which can continuously produce a reinforced plastic pipe having a spigot element with high strength and a desired outside diameter and a desired outside surface shape.